The present invention relates to optical device fabrication and, more particularly, to the utilization of a focused ion beam technique to form optoelectronic devices with angled facets that exhibit minimal light scattering.
Semiconductor optical devices such as lasers, superluminant diodes, modulators and amplifiers typically use cleaved ends of the semiconductor crystal as substrates upon which coatings are applied to form mirror and/or antireflection coatings. In the case of a Fabry-Perot diode laser, the perpendicular orientation of the facets with respect to the laser waveguide provides a convenient mirror orientation which efficiently couples light back into the resonator cavity. In the case of other types of lasers, as well as amplifiers and superluminant diodes, the smooth cleaved facet produces very little light scattering which, in conjunction with antireflection coatings, produces the very low feedback levels crucial for optimal performance of these devices.
It is difficult to create antireflection coatings with the extremely low reflection coefficients needed for proper laser amplifier and superluminant diode performance through the conventional process of deposition of dielectric layers on facets oriented perpendicular to the device waveguide. In an effort to reduce reflections from an imperfectly coated facet, devices have been fabricated with the waveguide oriented at an angle off-normal with respect to the cleaved facet. With this orientation, the majority of any light reflected from the facet will not re-couple back into the active waveguide. This method is effective in reducing reflectivity, but has the disadvantage that the process results in angling both the front and rear mirrors; thus producing lower reflectivities at both facets. This lower-than-desired reflectivity of the rear facet leads to lower-than-optimal output power from superluminant diodes. In general, it would be desirous to have the rear facet normal to the device beam mode, while having the front mirror angled. To this end, devices have been produced wherein the waveguide was bent so that while both facets are cleaved, one end of the waveguide terminates perpendicular to a cleaved facet while the other end, because of the bent waveguide, terminates at an angle with respect to the waveguide. In these cases the fabrication of the angled or bent waveguides has been problematic. Another approach has been to make devices in which one facet has been etched at an angle using a mask on top of the device and reactive ion beam etching or chemically-assisted ion beam etching the facet as discussed in the article "Superluminescent Diodes with Angled Facet Etched by Chemically Assisted Ion Beam Etching", by C. F. Lin appearing in Electronics Letters, Vol. 27, No. 11, 1991, at p. 968. In particular, the article discusses removal of material in a plane extending vertically downward from the edge of the mask. This process is made difficult by the care needed to establish etching conditions that remove material straight down the desired vertical plane below the mask edge. Furthermore, serrations along the edge of the photolithographically defined mask are replicated down into the etched facet, producing striations and roughness that scatter light back into the active waveguide. This scattering, as mentioned above, reduces the antireflection property of the angled facet, thereby mitigating its usefulness.
It has been suggested by Harriot, Scotti et al. in an article appearing in Applied Physics Letters, Vol. 48, No. 25, 1986, at pp. 1704 et seq. that out of-plane light beams may be produced by etching wedge-shaped recesses into the device substrate by making cuts with a focused ion beam directed normal to the wafer surface such that the total dose delivered to each location defines the depth of the cut at that location. One wall of the wedge-shaped recess serves as the laser mirror while the other sloping side of the wedge acts as a turning mirror directing the light out of the plane of the substrate. A demonstration of this method was implemented using a focused ion beam to cut a series of layered rectangular cuts, with the cut in each successive layer being slightly smaller than the previous. In this way, a very finely-divided staircase may be etched into the device, producing an essentially vertical wall and an essentially sloping surface. The disadvantage of this technique is that such surfaces are not particularly smooth and, therefore, scatter light undesirably. In addition, this technique is sensitive to inhomogeneities in the material density, phase, and crystallographic orientation (for polycrystalline samples) which translate into unexpected fluctuations in the depth of the cut. This again results in a rough and uneven mirror surface which scatters light. Backscattered light re-entering the laser or amplifier constitutes unwanted reflectivity and compromises the performance of the device. Likewise, forward scattered light from different positions of the mirror surface will be out of phase with the specularly reflected light from the mirror and therefore contributes to spatial inhomogeneities in the phase of the outgoing wavefront (degrading the reflected beam quality).
Thus, a need remains in the art for improved methods of facet etching that produce minimal reflection of light in unwanted directions.